Origin of Band Modulation in GeTe-Rich Ge−Sb−Te Thin FilmDeniz P. Wong,*,†,¶,◐ Masoud Aminzare,†,◐ Ta-Lei Chou,† Chin-Sheng Pang,‡ Yi-Ren Liu,‡

Tzu-Hsien Shen,† Benjamin K. Chang,† Hsiang-Ting Lien,‡ Sun-Tang Chang,∥ Chia-Hua Chien,⊥

Yang-Yuan Chen,⊥ Ming-Wen Chu,‡,§ Yaw-Wen Yang,∥ Wen-Pin Hsieh,# Gerda Rogl,∇,○

Peter Rogl,∇,○ Yohei Kakefuda,◆ Takao Mori,◆ Mei-Yin Chou,† Li-Chyong Chen,*,‡,§

and Kuei-Hsien Chen*,†,‡

†Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan‡Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan§Center of Atomic Initiative for New Materials, National Taiwan University, Taipei 10617, Taiwan∥National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan⊥Institute of Physics, Academia Sinica, Taipei 11529, Taiwan#Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan∇Christian Doppler Laboratory for Thermoelectricity, Institute of Materials Chemistry, University of Vienna, Wahringerstrasse 42,A-1090 Wien, Austria○Institute of Solid State Physics, TU-Wien, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria◆International Center for Materials Nanoarchitectonics (WPI-MANA) and Center for Functional Sensor & Actuator (CFSN),Namiki 1-1, Tsukuba 305-0044, Japan

*S Supporting Information

ABSTRACT: Germanium tellurides and their pseudobinary compounds offerinteresting properties that are important in thermoelectric and phase-changeapplications. Despite being a class of materials under scrutiny since its discovery,unique properties and functionalities have kept on emerging in recent years. Inthis work, we observed another unique property of Ge−Sb−Te (GST) thin filmthat can be beneficial in its development for thermoelectric applications. A rapidheating and quenching process of the GST film resulted in a metastable rock-saltcubic structure, exhibiting a unique electronic-transition-like behavior. Above thetransition temperature at 150 °C, we observed a temperature-induced bandmodulation, corroborated with changes in its effective mass and valence bandposition that leads to favorable electronic and thermoelectric properties. Chargetransfer between Sb and Te occurred, accompanied by a distorted cubic-to-cubicstructural change. The interplay of the electronic and lattice structure born out ofthe composition and phase of the Ge−Sb−Te thin film opened up the possibilityfor the future of thermoelectric devices.

KEYWORDS: band modulation, electronic transition, germanium tellurides, phase-change, thermoelectric

■ INTRODUCTION

Germanium tellurides and their pseudobinary counterpartshave long been used in electronic-related devices. Specifically,the report of S. R. Ovshinsky in 19681 on a semiconductorbehavior that can easily cross-over between a highly resistivestate and a conducting state has placed this class of materialsunder the scrutiny of various scientific curiosities about itsproperties and functionalities. Ge−Sb−Te (GST) has garneredmost of its prominence in phase-change memory applicationssuch as optical storage devices and memory devices.2−6

Recently, its applications extended into photonics,7,8 plas-monics,9 and even thermoelectric applications.10,11

The increasing possibilities of this class of material havemotivated many scientists to continue to study and understandthe underlying properties it can exhibit.12 Most of thethermoelectric studies for GST-related compounds focusedon bulk systems with the highest reported zT value of about2.4.13−17 In the case of bulk compounds, its thermoelectricproperties are tunable by varying composition,18 micro-structure,19 and vacancy content.20 The highest thermoelectricperformance occurs when the material is GeTe-rich in

Received: September 13, 2019Accepted: November 19, 2019Published: November 19, 2019

Article

pubs.acs.org/acsaelmCite This: ACS Appl. Electron. Mater. 2019, 1, 2619−2625

© 2019 American Chemical Society 2619 DOI: 10.1021/acsaelm.9b00596ACS Appl. Electron. Mater. 2019, 1, 2619−2625

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composition and transforms into its higher-temperature rock-salt cubic structure.21−23 This phase is interesting because, inaddition to possessing a higher thermoelectric performance,thin films of a GeTe-rich GST phase also stabilize in this cubicstate even at room temperature.24,25 The stark contrastbetween the bulk and thin-film form of this compoundmotivated us to look into the latter form for a possibleenhancement in its thermoelectric properties.The variation between the bulk and thin-film version of the

same compound is not something surprising. Due to thefabrication process, different microstructures can form fromvarying nucleation and growth processes.26 Furthermore, thepresence of another interface such as the substrate contributesan additional external effect creating possible strain and avarying mass diffusion process.27,28 Earlier reports on thethermoelectric studies of GST films have shown promise inachieving a better thermoelectric performance compared to thereported bulk counterparts at the same operating temperaturerange. However, these works investigated only a narrowoperating temperature range.29 In contrast, our current workshowed that, by going beyond this temperature, we observed aunique transition temperature, albeit with appropriate processconditions, which can improve its electrical conductivitywithout sacrificing its Seebeck coefficient. Recent studies onGST systems have suggested a disorder−order transformation-induced vacancy ordering, consequently influencing theirelectronic properties.30−32 Although it is still debatable, ourcurrent work demonstrated that such electronic-transition-likephenomena can ultimately lead to an improved thermoelectricperformance. The accompanying in situ observation of thelattice and electronic structure at the transition temperaturegave strong evidence on the possible origins of such bandmodulation.

■ RESULTS AND DISCUSSIONPreviously, we also have shown that the thermoelectricproperties of GST in the thin-film form were tunable throughmorphological control and different from its bulk counter-part.33 Here, we further demonstrate that a nonequilibriumthermal treatment, in which the film was rapidly heated (at arate of ∼50 °C/min) up to its decomposition temperature(∼550 °C) followed immediately by fast air-quenching,allowed us to obtain a nonstoichiometric GeTe-rich GSTthin film. The quenching process is needed to maintainstability of the film otherwise the interdiffusion of the Sb andTe elements as reported by Lee et al.34 causes significantmodification to the film structure and composition. Theprocess allowed us to maintain the initial composition withinour measurement temperature region (Figure S1). Afterobtaining the crystalline phase through this process, noadditional or changed structural phase was observed (FigureS2). All the diffraction peaks can be identified as rock-saltcubic structure with no rigorously defined phase transition.Furthermore, there is no sign of significant preferredorientation of the film, and the small crystallite size (40 nm)calculated based from the Scherrer equation suggests that thefilm is polycrystalline in nature and that a similar electronic andthermal transport behavior in both the in-plane and out-of-plane direction would be expected.Looking at the temperature-dependent thermoelectric

parameter values, the improved performance comes from anumber of favorable conditions such as a rapid increase inelectrical conductivity (Figure 1a) while keeping a relatively

temperature-independent high Seebeck coefficient (Figure 1b)above 150 °C. This allowed us to obtain an average powerfactor around 54 (±9) μW/cm K (Figure 1c). A high value ofthe power factor indicates that a TE device with this materialcan extract more energy from the temperature gradientgenerated. We also corroborated our measurements fromanother group using their own ZEM-3 system (Figure S3) andobtained agreeable results. At the same time, a similar trendwas also achievable on a different sample with variedcomposition (Figure S4). To evaluate the contribution fromthe substrate, measurement on a blank substrate was carriedout as well (Figure S5). As revealed by the negative Seebeckcoefficient, the intrinsic Si substrate carries n-type carriers withvery low electrical conductivity consistent with the reported

Figure 1. Thermoelectric properties as a function of temperature forGe−Sb−Te thin films. (a) Electrical conductivity. (b) Seebeckcoefficient. (c) Power factor.

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behavior.35 Thus, this unique electronic feature can beattributed solely to the GST film.Applying the Arrhenius equation to the temperature-

dependent electrical conductivity data, two activation energiescan clearly be distinguished below and above the transitiontemperature (Figure 2a). The activation energy observed

above the transition temperature is higher compare to its lowercounterpart plausibly due to different types of operatingcarriers and can be attributed to a heavy-hole carrier typicallygenerated in high-zT thermoelectric materials at a highertemperature as reported by Biswas et al.36 In order to confirmthis behavior, we performed a temperature-dependent Hallmeasurement on the sample to determine the Hall coefficient(Figure S6), carrier concentration, mobility, and effective mass(Figure 2b). The rapid change in all these physical parametersabove the transition point can be attributed to the contributionof another band as observed by Wu et al. on the bulk GeTe-based system.37 This phenomenon was observed throughoutdifferent batches of samples (Figure S7). The temperature-dependent effective mass, m*, was obtained using eq 1:

π πη

= *SK

ehm T

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where S stands for the Seebeck coefficient, KB is the Boltzmannconstant, e is the elementary charge, h is Planck’s constant, m*is the effective mass, T is the temperature in Kelvin, and η isthe carrier concentration. Notably, an abrupt multifold increasein the effective mass was observed above 150 °C, which impliescertain temperature-induced modulation in the band structure,similar to the band convergence phenomenon reported by Peiet al.38 and Liu et al.39 occurring in a different but high-performance thermoelectric material system.In order to investigate the origins of the temperature

evolution of the energy levels, we performed a temperature-dependent photoelectron spectroscopy study on the film.Ultraviolet photoelectron spectroscopy (UPS) allows us toprobe the valence band region and work function of thematerial. In contrast, X-ray photoelectron spectroscopy (XPS)allows us to probe the core-level region and observe thechemical and electronic states of the elements within thematerial. The valence band spectra of Ge−Sb−Te observed inFigure 3a contain two peaks around 0.75 and 2.75 eV. Theseare consistent features attributed to Ge 4p, Sb 5p, and Te 5porbitals of Ge−Sb−Te as demonstrated by Kim et al.40

Moreover, by obtaining temperature-dependent spectra, Figure3a shows a shift in the valence band maximum of about 0.25eV toward the vacuum level when heated above 150 °C(transition temperature). The upshift of the valence band isconsistent with the generation of the second carrier above thetransition temperature and indicates a plausible convergence ofband in the material. Most reported band convergencephenomena occur when the chemical structure is tunable bydoping or alloying,41 whereas we observed a temperature-induced band convergence-like behavior in Ge−Sb−Te abovea transition temperature of 150 °C. The directly measured shiftin the valence band by UPS, together with the aforementionedchange in the effective mass, concertedly implies a valenceband modulation, allowing the material to gain access to theelectronic state from another band below its original valenceband position as shown from ab initio calculations (Figure S8).The ground-state band structure of the GeTe crystalcontaining both a Ge vacancy and a Sb atom substituting anearest-neighbor Ge atom in a 4 × 4 × 4 supercell hassuggested no significant change in the band dispersions. Thetemperature-dependent band structure calculation is still notpossible at the present time. The full-scale UPS spectra (FigureS9) show that there are no significant changes in our workfunction; thus, the temperature-dependent band modulationcan be attributed to the changes observed in the valence bandregion.Furthermore, XPS data (Figure S10) have shown no changes

in the germanium peak while a shift toward the lower bindingenergy was notable for the tellurium peaks and a higherbinding energy for the antimony peaks. To get a better peakquality, the Sb 3d and Te 3d peaks (Figure S11) were alsoobtained, and a similar trend was observed (Figure 3b,c),suggesting a charge transfer effect between Sb and Te. Theoccurrence of the shift in these two peaks also coincides withthat of the transition temperature observed in electricalconductivity. Such an effect causes the changes in theelectronic transition as observed in the band structure,consequently, the increase in the power factor. The resonantbonding model, used to describe the local structure of the GSTsystem42 also reinforces the plausibility of a delocalization ofelectrons through the alignment of the p-orbital between theSb and Te atoms.43,44

Figure 2. Electronic transport properties of Ge−Sb−Te thin films. (a)Arrhenius plot of electrical conductivity. (b) Carrier concentration,mobility, and effective mass.

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Besides observing the changes in the electronic structure ofthe material, we also monitored the structural evolution nearthe transition temperature. Despite no change in the phasepresent, apparent red shifts in peak positions and, therefore, anincrease in the corresponding lattice constants (Figure 4) wereobserved. This can also be supported by the changes in thetemperature-dependent selected area electron diffraction(SAED) obtained from our TEM analysis (Figure S12). Itshould be emphasized that, by comparing with known thermal

expansion coefficients from related materials, the noteworthylattice expansion observed in our GST thin film does notfollow the thermal expansion coefficients of the bulk GeTe orGe2Sb2Te5 materials, or the substrates (Figure S13), of whichthe changes are negligible over the temperature range ofinterest. However, as we scratched the GST from the substrateand performed the temperature-dependent XRD measurementon the powdered sample, we observed an almost similarbehavior to the bulk system. This unusual behavior of the GSTthin film continues to persist as observed from the disparity oflattice constants at room temperature calculated from differentorientations. This is in contrast to most known cubic structuresand with the powdered sample obtained from the scratchedfilm (Figure S14). With increasing temperature, specificallyabove the transition temperature, the lattice constant valuesconverged, indicating an ordering of the crystal structure. It isknown from the literature that the rock-salt cubic structure is ahigh-temperature phase, and such a structure can be sustaineddown to lower temperatures in a metastable distorted form.45

With a further increase of the temperature, a perfect cubictransformation was observed, with the same lattice constantvalues calculated from two different crystal orientations (200and 220). Furthermore, this effect is reversible as indicated bythe lattice constants obtained from several heating and coolingcycles (Figure S15). This slight structural transformation mayinduce strain energy in the material system that leads tochanges in its electronic properties.46 Similarly, by observingthe evolution of the film in situ under the electron microscope,we observed the presence of a stacking fault that typically ispresent for a cubic system such as GST (Figure S16). Theslight movement of the stacking fault might influence the localchemical environment and lead to a considerable increase inthe electrical conductivity of the film.47

For the first time, by monitoring both the structural andelectronic structures of the film at the measured temperaturerange, we were able to observe the possible origins of the bandmodulation wherein a distorted-to-order structure wasobserved that causes a charge transfer effect between the Sband Te. The transition caused by such phenomena allowed thesudden increase in our film’s electrical conductivity thusboosting further its power factor and consequently plausibly itszT value. Such observation opens up an avenue of further

Figure 3. Photoemission spectroscopy as a function of temperaturefor Ge−Sb−Te thin films. (a) Temperature-dependent valence bandspectra and work function using an excitation energy of 40 eV. Theenergy positions at the valence band edge were taken to be theintersection points of the fitted lines labeled “x” and “y” on thespectrum. (b) XPS binding energy shift obtained from Sb 3d (FigureS10a,b). (c) XPS binding energy shift obtained from Te 3d (FigureS10c,d).

Figure 4. Lattice constant as a function of temperature of Ge−Sb−Tefilm obtained from the XRD pattern calculated using the (2 0 0) and(2 2 0) position.

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development for this class of material in the future and offersinsight into a different origin of electronic band modification.

■ CONCLUSION

The temperature-induced structural ordering leading bandmodulation of the GST thin film has shown a unique propertythat can boost the thermoelectric performance. Withoutobserving any phase change, we have demonstrated that,above 150 °C, the band modulation leads to favorablethermoelectric parameters. In this transition state, we alsoobserved a charge transfer between the antimony and telluriumatoms and in parallel a distorted cubic to an ordered cubictransformation. These findings not only provide intriguinginformation on the known phase-change characteristics butalso give new perspectives for the thermoelectric properties ofGe−Sb−Te and the development of new functional nano-devices.

■ EXPERIMENTAL METHODSThin-Film Fabrication. Ge−Sb−Te thin films were deposited on

intrinsic single crystal ⟨100⟩ Si substrates (electrical resistivity ≥3000Ω cm, Semiconductor Wafers, Inc.) using a radiofrequency magnet-ron sputtering system, with a base pressure <10−6 Torr. Commercial 2in. Ge19Sb2Te22 target (99.9%, Toshima, Japan) was used for thisstudy. Before deposition, the Si substrates were cleaned using astandard RCA process. The target was kept facing the substrate holder≈12 cm away. In order to deposit uniform films, the substrate wasrotated with a fixed speed throughout the deposition. Sputter-etchesof 5−10 min were used to remove the target surface contaminations.The target power, argon flow rate, and working pressure were fixed at80 W, 2 sccm, and 1.3 mTorr, respectively. The deposition time was18 min, and the resulting film thickness was approximately 250−300nm, estimated from scanning electron microscopy (SEM, JEOL JSM-6700F) cross-section images. The as-deposited film has an amorphousfeature and requires a postannealing treatment, by heating to 550 °Cat a heating rate of 50 °C/min under Ar atmosphere without holdingtime. The plan-view and cross-section images of the film are availablein the Supporting Information (Figure S17).Electrical Transport Properties. The temperature-dependent

electrical conductivity and Seebeck coefficient were obtained with thecommercial ZEM-3 system by ULVAC using a special thin-filmholder. Typical thin-film samples used for these measurements werecut into 10 mm × 3 mm slivers. Temperature-dependent Hall effectmeasurements (to obtain carrier concentration and mobility) wereobtained with the commercial ECOPIA HMS-3000. The sample usedfor these measurements had the size of about 15 mm × 15 mm.Structural and Composition Characterizations. The compo-

sitions, as summarized in Table S1, were checked using an electronprobe microanalyzer (EPMA), or wavelength-dispersive spectroscopy(WDS), attached to an SEM (JEOL JXA-8200 Electron Probe X-raymicroanalyzer) and confirmed by inductively coupled plasma opticalemission spectrometry (ICP-OES, Agilent 725) and X-ray photo-electron spectroscopy (XPS, NSRRC BL024A).Structural analysis was done using temperature-dependent XRD

(Pananalytical Empyrean) with Cu Kα1 radiation (λ = 0.154 nm). Aheating stage inside the X-ray diffractometer controls the temperaturefrom room temperature to 350 °C with an increment of 50 °C foreach measurement. Temperature-dependent changes in the latticeconstant through the selected area electron diffraction (SAED) werecrosschecked using a transmission electron microscope (JEOL, JEM-2100) equipped with a heating stage.Synchrotron-Based Study. Photoemission spectroscopy was

performed in the National Synchrotron Radiation Research Center(NSRRC), Hsinchu, Taiwan, at beamline 24A. This beamline offers a1.5 GeV bending magnet in a wide range spherical gratingmonochromator based on the Dragon type design giving a photonflux of 1 × 1010 to 5 × 1011 photons/s. The samples were loaded in a

heating stage and evacuated to a base pressure of 1 × 10−9 Torr. Priorto data acquisition, an Ar ion-sputtering gun was used to clean thesample surface by removing oxides and unwanted substances. For theultraviolet photoemission spectroscopy (UPS), the electron energywas measured with a PHOIBOS 150 energy analyzer (SPECSGmbH) equipped with a charge-coupled device detector housed in amagnetically shielded mu-metal chamber. The UPS excitation energywas 40 eV, and the Fermi edge was calibrated to a cleaned Mo surface.The total energy resolution was estimated from the Fermi cutoff ofthe Mo surface to be 100−200 meV at room temperature (RT)depending on the experimental conditions. A sample bias of −5.0 Vwas applied during the measurements in order to distinguish betweenthe analyzer and the sample cutoffs for work function determination.For the X-ray photoemission study, an excitation energy of 750 eVwas used. Similarly, the sample surface was cleaned prior to theexperiment. The temperature-dependent component of the instru-ment used a resistive-heating function, and the temperature wasmonitored using a thermocouple attached to a digital thermometer(accuracy of ±10 °C).

Computational Methods. First-principles calculations wereperformed to investigate the electronic structure of a GeTe crystalin the presence of Ge vacancies and/or Sb substitution. Differentconfigurations in a 4 × 4 × 4 supercell containing a Ge vacancy and/or a Sb atom replacing Ge were considered. We used the Vienna abinitio simulation package (VASP)48,49 code with projector augmentedwave (PAW) pseudopotentials.50,51 The generalized gradientapproximation (GGA)52 was used for the exchange-correlationfunctional, and a Γ-centered 3 × 3 × 3 k-mesh for the supercellwas adopted. We first carried out volume-constrained structuralrelaxation to determine the atomic geometry and then performed self-consistent electronic calculations including spin−orbit coupling(SOC). To compare the band dispersions in the presence of Gevacancies and/or Sb substitution with those in the perfect GeTecrystal, we used the zone-unfolding technique53 as implemented inthe BandUP54−57 code to represent the calculated band structureobtained using the supercell back in the Brillouin zone of the primitivecell.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsaelm.9b00596.

Sample compositions, additional temperature-dependentelectronic transport properties and validation, Hallcoefficient, carrier concentration, band structure fromcalculations, raw data of UPS and XPS, temperature-dependent TEM, comparison of lattice constants, andSEM images (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*Phone: +886 2 33665200. E-mail: deniz.wong@helmholtz-berlin.de.*E-mail: chenlc@ntu.edu.tw.*E-mail: chenkh@pub.iams.sinica.edu.tw.ORCIDDeniz P. Wong: 0000-0001-7126-0019Chin-Sheng Pang: 0000-0001-8347-4102Peter Rogl: 0000-0002-7733-1612Takao Mori: 0000-0003-2682-1846Present Address¶D.P.W.: Dept. of Methods for Characterization of TransportPhenomena in Energy Materials, Helmholtz-Zentrum Berlinfur Materialien and Energie, D-14109 Berlin, Germany

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Author Contributions◐D.P.W. and M.A. contributed equally to this work.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by the Thermoelectric Project (AS-106-SS-A01) funded by Center for Sustainability Science,Academia Sinica. The authors also acknowledge the technicalsupport provided by the Nano Core Facility of AcademiaSinica, Taiwan.

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